The present invention relates to a novel flexible duct structure without appreciable variation in length under the effect of an internal pressure.
The invention applies more particularly to constructing flexible ducts for transporting pressurized fluids, such as water or hydrocarbons.
For numerous applications of such ducts, it is often desirable, and sometimes necessary, to have available flexible ducts which show no appreciable variation in length when they are subjected to an internal pressure with "bottom effect", namely, the application of an axial force resulting from the action of the internal pressure at the ends of the flexible ducts.
In fact, extension of the flexible duct in the service, under the effect of the internal pressure, risks being a serious drawback in numerous applications.
This is a case more especially for sub-sea collecting ducts which must be embedded, i.e., disposed at the bottom of a trench, so as to avoid risks of damage due to shipping.
If the duct thus disposed tends to grow longer when it is pressurized, it risks being deformed vertically and to reappear out of the trench, which obviously cancels out the embedding effect.
In, for example, French Patents 1,417,966 and 2,177,966, structures of "multi-layer" reinforced flexible ducts are proposed to which the improvement of the invention applies, with the flexible ducts mainly comprising a pressure resisting reinforcement comprising one or more layers of rings or wires, cables or extruded sections wound at a high angle with respect to the axis of the flexible duct, which angle might possibly exceed 85.degree.. A traction resistant reinforcement is provided which comprises one or more pairs of crossed layers of wires, cables or extruded sections wound in an angle of at most 55.degree.; however, the angle is not to be too small so as to maintain the flexibility of the duct.
These above noted different layers may be disposed in any order from the inside to the outside of the flexible duct structure and be separated or not by plastic sheaths of small thickness.
U.S. Pat. No. 2,727,616 proposes adopting an angle greater than 55.degree. for winding the helically disposed fibers so as to overcome the problem of increasing the bending stiffness, but this patent relates to the construction of a rigid pipe and not a flexible tube.
The stiffness K of a reinforcement, proportional to the modulus of elasticity E of the material forming the layers thereof and to the amount of this material is defined by the product EQU K=E.multidot.e
where e, representing a total useful thickness, is, in fact, the total thickness of the layers of the reinforcement when these layers are formed from solid rectangular and jointing wires. When it is a question of hollow or round wires or extruded sections, the useful thickness is deduced from the total thickness e.sub.g taking into account the empty spaces or clearances formed inside the wires or therebetween, the relationship is: EQU e=b.multidot.e.sub.g
where b represents the ratio of the volume of reinforcing material forming the reinforcement to the total volume occupied thereby.
The index p is attributed to the reinforcement withstanding the internal pressure and t to the reinforcement withstanding the traction.
Thus, EQU K.sub.p =E.sub.p .multidot.e.sub.p and K.sub.t =E.sub.t .multidot.e.sub.t.
In flexible ducts of the prior art, in which it is desired above all to distribute the stresses in the different reinforcement layers so as to reduce the total thickness and so the weight and price of the flexible duct, the ratio K.sub.t /K.sub.p is small (less than unity).
But this type of structure cannot lead to flexible ducts which are stable in length under the effect of the internal pressure. In the present state of technique, in order to reduce the extension as much as possible, the traction resisting reinforcement is wound at a very small angle (&lt;20.degree.) which causes greater manufacturing difficulties but does not reduce this extension sufficiently for numerous applications, particularly in sub-sea collecting ducts which may be embedded.
To avoid variations in length under the effect of internal pressure, in accordance with advantageous features of the present invention, a flexible structure of the above-mentioned type is provided but the ratio of stiffness K.sub.t /K.sub.p is between 2.5 and 8 for a winding angle of the traction reinforcements .alpha. between 20.degree. and 50.degree..
It has been determined that a better result is obtained when the ratio of stiffness and the angle .alpha. are linked by the following relationships: ##EQU2## where .alpha., expressed in degrees is between 20.degree. and 50.degree..
In order to reduce the influence of an error on the winding angle, that is to say increasing the acceptable tolerance concerning this manufacturing parameter, a structure will be preferred such that the following relationship exists ##EQU3## where .alpha. in degrees is limited between 25 and 45.
Thus, by forming a flexible duct such that the ratio of the reinforcement stiffnesses K.sub.t /K.sub.p =3, where the traction layers are wound at 35.degree., a variation in length under pressure is obtained which is at least ten times less than would be the case at present with conventional structures, while allowing a tolerance of 10% in the ratio of the stiffnesses and from 2.degree. to 3.degree. in the winding angle.